When an alternating electromagnetic field, such as a high-frequency microwave, interacts with a fluid, a host of dynamic polarization processes take place within the fluid. These processes depend, in part, on the complexity of the fluid and on the physical properties of the components (e.g., the complex permittivity) that make up the fluid. Dielectric relaxation due to the electric dipole re-orientation of the fluid atoms and/or molecules, in addition to ion conductivity effects, manifest themselves in the frequency dependence of microwaves that are transmitted or reflected from the sample.
The measured dielectric response is enhanced when the fluid under test passes through, or is housed within, a resonant cavity tuned to the desired measurement frequency, a process known as cavity-enhanced dielectric relaxation spectroscopy. Cavity-enhanced dielectric relaxation spectroscopy at high frequencies typically employs a metal-walled cylindrical resonator as the microwave cavity. The geometry of the resonator is chosen according to the desired operating frequency. Accordingly, the inner diameter of the cavity and, thus, the space available for a fluid sample under test is constrained by the chosen operating frequency. In a conventional metal resonator operating at 18 GHz, the peak of the resonant dielectric relaxation response of water, the presence of an opening for the fluid flow line or channel and the ohmic losses at the cavity metal walls limit the quality factor (Q-factor) of the cavity resonant mode to the order of 1,000. Dielectric interrogation of fluids at lower frequencies up to a few GHz typically does not employ resonant cavities but rather involve measurements across capacitor arms or measurements with electrodes invading the fluid volume.
Optical measurements are also deployed but, like low frequency microwaves, do not involve the use of cavity resonators. Optical measurements probe the much faster electronic transitions or molecular vibrational transitions of the sample.